If the block is real bad, you will want to smooth it down with some 80-100 grit paper, but by no means do you want to remove all the casting sand dimples. You will do a lot of extra work and the results won't be authentic, as has been stated.

Norman, I have a spare block for my 1912 van which has had a weld repair that required grinding. When I primed the repair I had to chuck a handful of sawdust into the wet paint to get the repair rough again.